1. Field of the Invention
The present invention relates to a polishing liquid composition for a magnetic disk substrate and to a method of producing a magnetic disk substrate using the polishing liquid composition.
2. Description of Related Art
With the recent advances in the reduction in size and the increase in capacity of magnetic disk drives, there is a need to increase the recording density. To increase the recording density, there have been developed techniques for further improving the detection sensitivity for a magnetic signal weakened due to a reduction in the unit recording area by reducing the flying height of magnetic heads. In order to take measures for reducing the flying height of magnetic heads and ensuring the recording area, the requirements for magnetic disk substrates are becoming increasingly stringent, with regard to the improvement of smoothness and flatness (the reduction of surface roughness, waviness, and roll-off) and the reduction of surface defects (the reduction of residual abrasive grains, scratches, protrusions, pits and the like).
To meet such requirements, methods of producing the hard disk substrates usually employ a multistage polishing system that includes two or more polishing steps in terms of improving both the productivity and the surface quality such as better smoothness and less scratches. Generally, a polishing liquid composition for finishing containing colloidal silica particles is used in the final polishing step of the multistage polishing system, in other words, in the polishing step for finishing in order to meet such requirements as reductions in the surface roughness, scratches, projections and flaws such as pits, and a polishing liquid composition containing alumina particles is used in prior polishing steps (referred also to as rough polishing steps) to the polishing step for finishing in terms of improving the productivity (e.g., JP 2005-63530 A and JP 11-010492 A).
However, the use of alumina particles as abrasive grains may lead to defects in the media due to texture scratches caused by the embedding of the alumina particles into the substrates. In order to solve this problem, a polishing liquid composition containing alumina particles having a certain particle size and silica particles having a certain particle size distribution is proposed (e.g., JP 2009-176397 A).
Further, when abrasive grains used in the rough polishing steps remain on the hard disk substrates, it may induce scratches and pits during the polishing process for finishing, so that the yields of the substrates may drop. In order to solve this problem, a polishing liquid composition containing an organic nitrogen compound having two or more amino and/or imino groups in the molecules is proposed (JP 2006-150534 A).
Furthermore, to modify the surface of the objects to be polished to adjust the polishing rate, polishing liquid compositions containing an organic nitrogen compound having two or more amino and/or imino groups in the molecules are proposed (JP 2008-252022 A, JP 2010-541203 A and JP 2005-515646 A).